Controlled differentiation of stem cells will provide correction of disregulated lymphopoiesis which leads to cancers such as lymphomas, myelomas and leukemias. In addition, capacity for quantitative and qualitative development of appropriate lymphocytes necessary for responding to other kinds of neoplastic tissue and restricting its growth will take place in in vitro environments where host stem cells can be induced to functional states required for immunotherapy. Experimental approaches are designed which simulate an appropriate environment for expanding and exploiting the immune system. Developmental changes in surface membrane molecules will be correlated with differentiation of functionally discrete, biophysically purified classes of lymphocytes and their precursor cells to provide the understanding of normal regulation of immunogenesis in vitro. Selective maturation, induction and interaction of functional helper, effector, suppressor or killer T lymphocytes and primary, secondary or memory B lymphocytes will eventually be used to repair immunodeficiencies and provide lymphocytes capable of restricting neoplastic growth in situ. Memory cells will then be induced which continually sustain this population for repeated host protection.